We sent out a little card with the invitations that told people how/where to RSVP online, but then we did not have to pay for envelopes or postage back.
Honestly, no matter how you do it, some people will simply not reply and you will have to follow up with them. Sad, but true. Call them, email them, whatever you need to do that works.
We sent our STD's 9 months early and our invites 3 months early. You have to balance too early (people aren't ready to deal with it yet) and too late (people don't have time to book a trip).